The principle of scalability, designed to provide different levels of rebuilding from a single bitstream received by a decoder, is already known. For example, it makes it possible to propose different levels of rebuilding of a video from the received stream. These levels of rebuilding depend especially on the complexity of the decoding operations to be applied.
This is the case, for example, in the current H.264 standard which proposes scalability based on a hierarchical approach with inter-layer prediction enabling limits to be placed on redundancy between layers (see for example “SVC-Amendment 3” of the H.264/MPEG-4 AVC standard). The same information is represented at different hierarchical levels with a progression in quality and/or in resolution.
The SVC encoder proposes scalability in the sense that each decoder can choose the quantity of data that it will process as a function of the hierarchical levels that it chooses. The number of information pieces to be decoded can thus be reduced, of course with a trade-off in terms of deterioration of the quality of images rendered.
In order to limit the complexity of decoding of a stream according to a video standard (for example H.264), certain special applications provide for redefining certain decoding operations on the basis of less complex tools. However, this redefinition of the tools used in decoding must be taken into account in the process of encoding of the stream in order to prevent the decoder from drifting. Drift is the desychronization of the decoded images which act as reference images for the temporal prediction between the encoder and the decoder. If these images are not identical at both the encoder and the decoder, then the' decoder rebuilds images with errors which accumulate in the course of time.